There are essentially two types of optical scanners, namely, scanners that move scan heads and/or associated optics over stationary samples and scanners that move the samples relative to stationary scan heads and/or optics. What both types of scanners have in common is the need to accurately determine and/or predict the position of the moving part relative to the stationary part. We discuss scanners in the context of scanning laser microscopes below. The inventive system is not, however, limited to use as part of a scanning laser microscope.
A scanning laser microscope is an optical scanner that is used to examine fine details of small objects. For example, the scanning laser microscopes are used to examine fluorescence from chemically-tagged biological samples such as cells, proteins, genes and DNA sequences. The microscope collects data from successive "pixels," which are dimensioned in microns. The applications of interest here are best performed with microscopes with relatively wide fields.
Known wide-field scanning microscopes typically scan a laser beam over a stationary sample in a predetermined pattern that is commonly referred to as a raster scan. These microscopes must precisely align the optics in a moving scan head with the beam from a stationary laser or, alternatively, carry the laser on the moving scan. A stationary laser can be aligned with a moving scan head only at relatively low speeds, and thus, the scan speed of the system is essentially limited. The alternative system requires a relatively large scan head to carry the laser, and the size and weight of the scan head effectively limit the scan speed.
The known wide-field systems typically collect data when the laser beam is scanned in one direction (the forward direction) over the sample. As the beam moves across the sample in the forward scan direction, data is collected at selected pixel locations that span a target image. The beam is then swept over the sample in the backward direction, in what is commonly referred to as a "flyback" scan, during which data is not collected. The laser beam is then advanced and directed along a next forward scan, during which data is collected at the same selected pixel locations. The locations at which data is collected in each scan must align to well within a pixel, in order to avoid a skewing of the image data.
A uni-directional system has a short duty cycle, since data collection is restricted to less than one-half of the total scan travel. Certainly one way to decrease the sample scanning time is to collect data in both scan directions. However, to do this a system must compensate for the differences in the movement of the system between the two scan directions down to the sub-pixel level. One known bi-directional scanning system uses different grating, or position encoding, geometries for the forward and backward scans. This increases the complexity and the cost of the system. Further, such a system does not compensate for environmental factors such as differences in the thermal expansion of system parts, which may affect the movements in the two scan directions differently. These factors are important when the system must coordinate, in each direction, pixel locations that have a resolution of a small number of microns.
Another way to increase the system scan speed is to increase the rate at which the system moves the sample relative to the optics or vice versa. If bi-directional data collection is performed, compensation for the differences between the movements of the system in the two scan directions must be done quickly and accurately, to ensure that data is collected at corresponding pixel locations in each direction.